It doesn't make a noticeable difference with external HDDs over USB 2.0, that's where the biggest bottleneck is.

edit for clarity: Data transfer in any system is only as fast as its slowest component. Most 5400RPM HDDs can give a transfer rate of over 1000Mb/s, but USB 2.0 has a maximum transfer rate of 480Mb/s. It doesn't matter that a 7200RPM HDD is even faster than a 5400RPM, the fact that it's going through USB means that the most you'll ever get is 480Mb/s regardless of the HDD.

Just copy a big file from the external HDD to an internal one on your pc and stop the time. Then compare your result with the ~8MB/s which is the theoretical maximum you can get with retail discs, and be happy.

It doesn't make a noticeable difference with external HDDs over USB 2.0, that's where the biggest bottleneck is.

edit for clarity: Data transfer in any system is only as fast as its slowest component. Most 5400RPM HDDs can give a transfer rate of over 1000Mb/s, but USB 2.0 has a maximum transfer rate of 480Mb/s. It doesn't matter that a 7200RPM HDD is even faster than a 5400RPM, the fact that it's going through USB means that the most you'll ever get is 480Mb/s regardless of the HDD.